An essential element of modern mobile communications systems is the cellular telephone base station, also known as a “cell site.” The cell site includes one or more directional base station antennas aimed at a desired geographical area of coverage with coaxial cables connecting the antennas to base station radio equipment. The performance of a cell site is often limited by passive intermodulation (PIM) interference. PIM interference occurs when the high-power downlink signals transmitted by the base station antennas mix at passive, non-linear junctions in the RF path, creating new signals known as intermodulation products. When these intermodulation products fall in an operator's uplink band, they act as interference and reduce the SINR (signal to interference plus noise ratio). As the SINR is reduced, the geographic coverage and data capacity of the cell site is reduced.
It is well documented that loosely touching metal-to-metal surfaces can behave in a non-linear fashion and become sources of PIM interference when illuminated by high power RF (radio frequency) signals. Recently, it has been determined that loose metal-to-metal connections located behind base station antennas are also able to generate high levels of PIM interference. Even though this region is well outside the main beam of the antenna, enough RF energy is present in this region to excite non-linear objects and generate PIM interference. Based on field measurements it has been determined that loose metal-to-metal contacts located very close to base station antennas (within one wavelength of the carrier frequency) are more likely to generate high levels of PIM interference than loose metal-to-metal contacts located farther away (greater than 1 wavelength) from base station antennas.
A common source of loose metal-to-metal contact found in the region close to the base station antenna is metal brackets and associated hardware for supporting coaxial cables. Coaxial cables, typically ½-inch in diameter, are used to transfer RF signals between tower mounted radio equipment and the base station antenna. These cables need to be mechanically supported periodically along their length to prevent movement of the cable in the wind. The metal antenna mounting pipe close to the back of the base station antenna provides a convenient rigid surface to mechanically secure these coaxial cables. At operating frequencies at or below 2 GHz, this mounting pipe is typically located within one wavelength of the antenna within the zone of high PIM concern.
Two different methods for mechanically supporting coaxial cables are commonly found at cell sites. The first utilizes two plastic clamp blocks that fit around one or more coaxial cables. An example of this style cable support block is disclosed in U.S. Pat. No. 5,794,897 to Jobin et al, which is incorporated by reference. A ⅜-inch or 10 mm diameter stainless steel threaded fastener is inserted through the support block pairs and stainless-steel hardware is installed to clamp the plastic block halves together on the threaded fastener. A steel interface bracket is often attached to one end of the threaded fastener using nuts and lock washers. The interface bracket is then secured to the antenna mounting pipe or other nearby metal members using a stainless-steel hose clamp. The hose clamp provides a convenient method for securing interface brackets to metal members since the hose clamp conforms easily to different shapes and is adjustable in length allowing it to fit around a wide variety of metal member sizes.
A second system for mechanically supporting coaxial cables uses metal “snap-in” style single-cable cable hangers. The snap-in cable hangers are made from thin “U” or “C” shaped stainless-steel members designed to wrap around individual coaxial cables. The hangers include locking features able to insert into round holes in supporting interface brackets. Once inserted, the locking features on the hanger expand outward to secure the cable to the interface bracket. Variations of this hanger design include holes on one end of the hanger to accept additional cable hangers. This enables multiple coaxial cables to be secured to a single interface bracket by stacking one hanger on top of another. Examples of this style cable hanger is disclosed in U.S. Pat. No. 6,899,305 to Korczak et al., which is incorporated by reference. The interface brackets used to support this style of hanger are also commonly secured to the antenna mounting pipe or other nearby metal members using stainless-steel hose clamps.
Multiple sources of PIM interference are present with these conventional cable support designs. First, if the hose clamp used to secure the interface bracket to the antenna mounting pole is not tightened sufficiently, PIM can be generated at the loose metal-to-metal contact between the hose clamp and the antenna mounting pipe and at the loose metal-to-metal contact between the interface bracket and the antenna mounting pipe. Second, even if tightened sufficiently, there is potential for PIM generation at the free end of the hose clamp beyond the tightening mechanism where the steel banding is able to lightly contact itself as well as other metal objects. Third, hose clamps are typically constructed from stainless-steel and antenna mounting pipes are typically constructed from galvanized steel. Galvanized steel and stainless steel are dissimilar metals at opposite ends of the galvanic series. Small pockets of corrosion can form over time between the two dissimilar metals that generate passive intermodulation. PIM can also be generated at metal-to-metal contacting surfaces between stacked metal snap-in style cable hangers and at the metal-to-metal contacting surface between the interface bracket and the snap-in hanger.
Manufacturers such as Commscope have recently introduced plastic versions of their stackable snap-in style cable hangers that will eliminate some of the PIM producing interfaces. U.S. Pat. No. 10,253,906 to Vaccaro, which is incorporated by reference, describes this product. The plastic snap-in style hangers do not, however, eliminate PIM associated with hose clamps or the PIM generated at the metal-to-metal contact between the metal interface bracket to antenna mounting pipe. In addition, FIMO, an Italian manufacture, sells the “BAP-10” plastic cable bracket, which eliminates some sources of PIM in the cable bracket interface. However, this device has an arcuate pole interface fitted to a specific pole size, which limits each cable bracket to a pole with a specific diameter.
Heavy duty plastic cable ties are available as a low PIM replacement for hose clamps for attaching metal interface brackets to metal support members at the cell site. These heavy duty plastic cable ties do not, however, eliminate PIM generated at the metal-to-metal contacts between the metal interface bracket and metal support members.
A reliable, inexpensive, easy to deploy cable support solution is needed to secure coaxial cables to antenna mounting pipes and other steel members in close proximity to base station antennas without generating passive intermodulation. It is desirable that this solution be able to re-use existing deployed hardware where possible to reduce cost.